1. Field of the Invention
The present invention relates to mass-transfer trays for chemical process columns and, more particularly, but not by way of limitation, to an improved liquid-liquid contactor tray for facilitating increased mass transfer efficiency.
2. History of Related Art
Distillation columns have been developed and used for many decades to separate selected components from a multicomponent stream. The major “separations” process is commonly known in the art as “fractionation.” Successful fractionation in a distillation column depends upon intimate contact between a heavier fluid and a lighter fluid. Some contact devices, such as, for example, trays, are characterized by relatively high pressure drop and relatively high fluid hold-up. One type of contact apparatus utilizes fluid in a vapor phase to contact fluid in a liquid phase. Another type of contact apparatus is structured packing. Structured packing is energy efficient as it exhibits low pressure drop and low fluid hold-up. However, these very properties at times make columns equipped with structured packing difficult to operate in a stable, consistent manner. Moreover, many applications simply require the use of trays.
A particularly effective tray in process columns is a sieve tray. Typically, the sieve tray is constructed with a plurality of apertures formed in a deck surface. The plurality of apertures permit ascending lighter fluid to flow into direct engagement with heavier fluid that is flowing across the sieve tray. When there is sufficient lighter-fluid flow upwardly through the sieve tray, the heavier fluid is prevented from running downwardly through the plurality of apertures (referred to as “weeping”). A small degree of weeping is normal in sieve trays while a larger degree of weeping is detrimental to the capacity and efficiency of the tray. Such trays may be either single-pass or multi-pass. In addition, such trays may incorporate serpentine flow, orbital flow, or uni-directional flow.
Another type of “separations” process involves mass transfer between two fluids which are both in a liquid state. This is commonly referred to as “fluid-fluid exchange.” The primary advantage of fluid-fluid exchange over fluid-vapor exchange is an amount of process energy required. In the fluid-vapor exchange, substantial energy must be provided and consumed to boil a fluid into a vapor state and maintain the fluid in the vapor state for the duration of the process. In contrast, most fluid-fluid exchange processes operate at temperatures slightly above ambient temperature such as, for example, 90° F. resulting in significant energy savings.
In cases involving fluid-fluid exchange, there are specific performance issues that impact efficiency. In typical fluid-fluid exchange columns, a first fluid is in a continuous phase and a second fluid is in a dispersed phase. In one arrangement, the heavier fluid, in a continuous phase, is passed downwardly in a circuitous path across a series of horizontally disposed trays spaced in a vertical relationship, one to the other. The heavier fluid forms a fluid layer on the trays. Droplets of the lighter fluid, in a dispersed phase, ascend through the plurality of apertures and interact with the continuous fluid. This arrangement may be used, for example, in re-capture of an acid where the heavier fluid is water containing the acid and the lighter fluid is a selected solvent. In another arrangement, the heavier fluid is the dispersed phase and the lighter fluid is the continuous phase. In this arrangement, the heavier fluid forms droplets which fall downwardly through the plurality of apertures. The heavier fluid droplets fall through the lighter fluid, in continuous phase, flowing upwardly in a circuitous path across an underside of the trays. This arrangement may be used, for example in solvent recovery of Benzene from aromatics process streams.
In conventional fluid-fluid contactor trays, velocities of the continuous-phase fluid are very low relative to fluid-vapor columns. The low velocities in the continuous-phase fluid result in the continuous-phase fluid having minimal head pressure thereby inducing re-circulation and stagnation. Recirculation and stagnation reduces mass-transfer driving force. Tray areas where no mass transfer between the continuous-phase fluid and the dispersed-phase fluid occurs are referred to as “dead zones.” Dead zones form in locations where the continuous-phase fluid stagnates thus exhausting the solvent absorption capability. Furthermore, the low velocities of the continuous-phase fluid result in the continuous-phase fluid tending to not cover an entire surface of a tray. Such incomplete tray coverage lessens an area of effective mass transfer and reduces an efficiency of the tray. These problems are generally present regardless of whether the heavier fluid or the lighter fluid is used as the continuous phase.
U.S. Pat. No. 7,556,734, assigned to AMT International Inc., teaches an exchange column for contacting liquid in a continuous phase with liquid in a dispersed phase. Contact between liquid in the continuous phase and liquid in the dispersed phase is enhanced by providing upstanding baffles on lower trays interspersed with depending baffles from trays above. In addition, flow distribution partitions extend along a flow path, between the baffles, to distribute liquid flow across the trays.
U.S. Pat. No. 4,247,521, assigned to Union Carbide Corporation, teaches a liquid-liquid contacting tray having a channelized liquid transfer means for transferring continuous phase liquid from a contacting zone on one side of the tray to a contacting zone on the other side of the tray. The channelized liquid transfer means includes a settling section operable to allow disengagement of the discontinuous phase liquid from the continuous phase liquid, and a pressure drop section.
U.S. Pat. No. 2,752,229 assigned to Universal Oil Products Company, teaches a tower for effecting countercurrent contacting of fluid streams, particularly liquid-liquid contacting. The tower includes a plurality of vertically spaced perforated liquid distributing decks extending across a confined chamber. Sloping liquid downpipe assemblies extend from a liquid receiving well on one deck to a liquid seal reservoir of the next lower deck. Use of the sloping downpipe ensures that the continuous-phase liquid moves in the same direction across successive trays thus creating a uni-directional flow path.